1. Field of the Invention
This invention relates to a motor vehicle transfer case; more particularly it pertains to a park-brake mechanism for an integrated transfer case and transmission.
2. Description of the Prior Art
A transfer case usually includes a planetary gear set for producing either a “high” range, in which the transfer case output is driven at the same speed as the input, or a “low” range, in which the output is driven slower than its input speed. The 4×2 and 4×4 states of the transfer case are usually selected manually by the vehicle operator by operating a lever or switch. A first position of the lever causes a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, the 4×2-drive mode. A second position of the lever causes the transfer case to direct power to both a front drive axle and a rear drive axle, the 4×4-drive mode.
In a driveline that includes a transfer case driven by a power transmission located in a torque delivery path between a power source, such as an engine or motor, and the drive wheels of the vehicle, a park-brake mechanism is provided to hold the transfer case output fixed against rotation. This mechanism prevents all the drive line components in the torque path from the location of the park mechanism to the drive wheels from rotating. The vehicle operator actuates this braking of the drive wheels by manually moving a gear selector to a park position.
However, failure of a component in the torque path between the park-brake mechanism and the drive wheels can result in loss of the park-braking function at the drive wheels. If such a failure occurs, the wheels can rotate and the vehicle can move even though the park mechanism and transmission are unaffected by the failure. Furthermore, although the vehicle operator can readily detect loss of the drive function because the wheels cannot be driven from the engine due to a failure in the torque path, the operator may not realize that the park function is also affected by a drive line component failure.
It is desirable to minimize the possibility of a drive line component failure that could affect the park-brake function.
High range and low range operation of the transfer case is produced conventionally by alternate engagement and disengagement of hydraulically actuated range clutches. The hydraulic clutches that control high and low range operation typically include a clutch pack of alternating spacer plates and friction discs, which are forced into friction contact when a piston located in a cylinder is pressurized with hydraulic fluid, thereby engaging the clutch. The clutch is disengaged by venting the cylinder, which permits a spring to release the piston allowing the plates and discs to separate.
In certain cases, at least one of the range clutches is normally-closed, requiring hydraulic pressure to disengage. This characteristic requires special precautions to assure that the normally-closed clutch will remain closed without hydraulic pressure during a park-brake condition. In other cases, where both range clutches are normally-open and because hydraulic pressure is usually unavailable during the park-brake condition, the park brake must bypass the range clutches and hold the transfer case output against rotation.
Additional security is provided if the range clutches are deleted entirely and replaced with a range selection device that provides a secure torque path compatible with the park-brake function, regardless of whether the high range or low range is selected. Preferably the range selection device is located in the transfer case, and the park brake holds the transfer case output against rotation on either the transmission case or transfer case. This arrangement brakes the drive wheels against rotation in the event of a failure of any component of the torque path between the engine and the transfer case output.